1. Field of the Invention
The present invention relates generally to the formation of composite structures and, more particularly, to the formation of substantially void-free laminate structures using inductive energy.
2. State of the Art
Composite articles are well known to provide advantages in diverse applications. In some applications, the advantages of composite articles over metal, ceramic or other materials include weight reduction and the ability to integrate several otherwise individual parts into a single structure. Composite articles may conventionally include a reinforcement material in a polymer-based resin, also known as the matrix material, such as thermoplastic or thermoset resin. The reinforcement materials may include, for example, chopped or continuous fibers disposed either randomly or in ordered fashion within the polymer matrix material. Composite materials are conventionally formed by configuring the reinforced matrix material into a desired form or structure, which may include heating the article to place the matrix material in a moldable condition followed by resolidification in the case of a thermoplastic, and curing of the matrix material in the case of thermoset resins.
Conventionally, composite structures are often formed as laminates, meaning that laminae, or multiple lamina, are layered on top of each other and bonded together by heating, thereby effecting melting in thermoplastics and effecting cross-linking between the multiple layers for thermoset resins. Additionally, a consolidation process is conventionally carried out on laminate structures to increase adhesion and reduce voids between the laminae. Such consolidation is conventionally carried out by processes such as vacuum debulk or through use of an autoclave.
One limitation associated with manufacturing composite articles or structures is the rate at which such articles may be processed, especially for composite articles exhibiting large cross-sectional areas. For example, existing techniques such as autoclave, pultrusion and double belt press techniques are limited at the rate by which thermal equilibrium can be achieved within the material during processing.
Conventionally, heating of composite articles has been carried out through surface heating techniques. However, the thicker the composite structure, the more difficult and time consuming it becomes to achieve the proper temperatures at or near the center of the structure. In order to reduce the process or cycle times of producing a laminate composite structure, the surface temperature may be increased in order to more quickly transfer thermal energy to the center of the composite structure. Referring to FIG. 1, an exemplary graph 100 shows the need to increase surface temperature of a composite structure in order to increase the throughput, or the amount of material processed in a given amount of time, in a conventional surface heating process. For example, the first plot 102 indicates that in order to maintain an exit temperature of 480° F. for a composite structure having a thermoplastic matrix, the surface temperature of the composite structure during processing must be increased approximately 1000° F. in order to realize a corresponding increase in the rate of throughput by approximately 19 feet/second (ft/sec). Similarly, as seen in the second plot 104, to maintain an exit temperature of 625° F. throughout the composite structure, an increase in surface temperature of approximately 1200° F. is required to increase the throughput by approximately 19 ft/sec.
However, the allowable surface temperature of the composite laminate structure is limited by its degradation temperature, which, in turn, limits the throughput or production rate. Thus, in using conventional surface heating techniques, the tradeoffs for improving production times include an increase in both capital costs and labor (i.e., through implementation of parallel production lines) and/or the production of a potentially degraded and inferior product.
In an attempt to improve production times, inductive heating techniques have been implemented in the production of composite structures. Induction heating techniques conventionally take advantage of the inductive transfer of energy from an induction coil to a conductive member either positioned adjacent a surface of the composite structure or disposed within the composite structure, such as between individual laminae or in the matrix material of an individual lamina.
For example, U.S. Pat. No. 5,229,562 issued to Burnett et al. discloses inductively heating a conductive member, such as a platen or a mandrel, which is positioned against a surface of the composite structure. However, such a method presents problems similar to those discussed above since the use of platens or mandrels is simply another means of surface heating the composite structure.
Another inductive heating technique includes placing a conductive member, such as a susceptor or a metal insert, into the composite and transferring energy through the insert and into the body of the composite. Such a technique, which may generally be referred to herein as volumetric heating, serves to bring the composite structure to thermal equilibrium much more efficiently. While volumetric heating brings the composite structure to thermal equilibrium much more quickly than surface heating, the use of metal inserts or susceptors to accomplish such may serve to mechanically weaken the resulting structure.
Another inductive heating technique involves transferring inductive energy to electrically conductive reinforcing fibers placed within the matrix material of a composite structure. For example, U.S. Pat. No. 4,871,412 issued to Felix et al., the disclosure of which is incorporated by reference herein, teaches the formation of local spot welds and seam welds for lap joints between two composite structures by inductively transferring energy to carbon fibers disposed within the two structures.
U.S. Pat. No. 5,357,085 issued to Sturman, Jr., the disclosure of which is incorporated by reference herein, teaches the heating of a polymer matrix composite strand by passing the strand through a helical guide tube adjacent an inductive coil to transfer energy into carbon fibers of the composite strand.
U.S. Pat. No. 5,338,497 issued to Murray et al., the disclosure of which is incorporated by reference herein, teaches forming a thick composite structure by placing the composite material into a mold and inductively transferring energy into conductive elements, such as metal whiskers, disposed in the matrix material while the composite material is in the mold.
However, while teaching volumetric heating of a composite structure, the above-referenced processes fail to address the use of volumetric heating in the high-volume production of laminate structures while retaining laminate quality, including the prevention of internal voids or warping. While conventional consolidation techniques may be used to reduce voids, i.e., through the use of an autoclave or by subjecting the structure to vacuum debulk, such techniques are time consuming and also require consumable or disposable waste materials. Such waste materials might include, for example, release films, bagging materials or nitrogen based volatiles used during pressurization.
In view of the shortcomings in the art, it would be advantageous to provide an apparatus and a method for forming laminated composite structures which allow for volumetric heating and prevent the subsequent growth of voids while also increasing throughput rates. It would be further advantageous to provide an apparatus and method which allow for continuous production of a laminate composite structure without the need for consumable waste materials conventionally used during consolidation.